In an electronic still camera 1 as shown in FIG. 10, an optical image formed by a lens 2 is incident to a solid state image pickup device (CCD: charge coupled device, or the like) 3, and the solid state image pickup device 3 optoelectrically converts the optical image to produce a charge image. The charge image is read over time, and then processed by a signal processing circuit to form a video image, which is recorded on a memory card 4.
Time setting (exposure time setting) of the electronic still camera is generally achieved by an electronic shutter action of the solid state image pickup device 3. The electronic shutter operation is for adjusting the storing time of the charge signal in the solid state image pickup device 3. More specifically, the CCD, for example, includes an optoelectric conversion unit (photodiode), a gate for reading electrical charges from the optoelectric conversion unit, and a charge transfer passage for transferring the read electrical charge. The electronic shutter operation can be carried out by adjusting a time interval from the time when light is incident to the optoelectric conversion unit to the time when the charge is read.
Some types of electronic still cameras include a mechanical shutter mechanism of the same type as a mechanical shutter used in optical cameras using silver halide films. However, most types use the above electronic shutter operation.
In order to achieve high image quality, some still video cameras include a multi-picture element CCD, which is frame driven and read by a domino method (described later). In this case, when light is incident to the CCD during transfer of the charge through the charge transfer passage, deterioration in image quality such as smears or white flaws are caused by the incident light. This phenomenon becomes conspicuous particularly when the charge during the transfer time exceeds 1V (vertical scanning time). In general, the charge transfer time becomes longer as the number of picture elements increases. Further, when light is incident to the CCD during the time when the charge is stationary (held) in the charge transfer passage, deterioration of image quality is small compared to the case where light is incident during charge transfer.
To prevent such deterioration in image quality, a shield plate has been used which is closed to block light from the lens to the CCD after the shutter operation. Such conventional methods include the following two types.
(1) After the charge is read from the optoelectric conversion unit to the charge transfer passage by the electronic shutter operation, the charge is once held within the charge transfer passage. The shield plate is closed during the holding and, after shielding is completed, charge transfer is begun.
(2) The shield plate is completely closed before the charge read from the optoelectric conversion unit to the charge transfer passage by the electronic shutter operation is begun to be transferred, that is, the shield plate is closed at a high speed in a short time (e.g. 0.6 msec).
However, in the conventional method (1) described above, since all of the charge is held until shielding is completed, the image quality tends to be deteriorated by a dark current.
Further, in the conventional method (2), since the shield plate must be closed at a high speed, a complex and expensive shield mechanism is required.
A primary object of the present invention is to provide an image pickup device shielding apparatus for an electronic still camera which provides a sufficient shielding effect with a simple shield mechanism while maintaining a good image quality.